1. Field of the Invention
The present invention relates to pseudo resistance circuits and charge detection circuits, and particularly relate to a pseudo resistance circuit including a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) that is included in a weak inversion region and a charge detection circuit including the pseudo resistance circuit.
2. Description of the Related Art
Recently, charge output sensors of charge generation type such as a strain gauge, an acceleration sensor, and the like using an insulator piezoelectric element have been used. Because such a charge output sensor detects a tiny amount of charge, an amplification circuit for amplifying the detection signal is needed.
Further, with semiconductor devices of today having highly sophisticated functions and being highly integrated, a resistance element with a resistance value on the order of giga-ohms is needed in the semiconductor devices.
Under the situations discussed above, Japanese Unexamined Patent Application Publication No. 2008-224230, which relates to a detection device, a sensor, and an electronic apparatus, discloses an amplification circuit including an operational amplifier whose non-inverting input terminal is grounded, and a resistance element and a capacitor that are electrically connected in parallel between an output terminal and an inverting input terminal of the operational amplifier.
International Publication No. 1995/25349, which relates to a giga-ohm load resistance for a microelectronic integrated circuit, discloses a technique for obtaining a high resistance element by using a MOSFET in a weak inversion region.
However, according to research conducted by the inventor of the present invention, in a charge detection circuit, there are many cases where a frequency range of a detection signal from a charge output sensor develops even to a low frequency region. In such a case, it has previously been necessary to provide a high resistance element of at least no less than tens of mega-ohms in order to lower the cutoff frequency that is defined by a resistance value of a resistance element and capacitance of a capacitor.
With the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2008-224230, if a high resistance element of no less than tens of mega-ohms is provided, the circuit configuration itself is caused to be large. Japanese Unexamined Patent Application Publication No. 2008-224230 does not disclose or implicitly describe, in any way, a specific configuration in which the circuit configuration is miniaturized and integrated.
Meanwhile, regarding the configuration disclosed in International Publication No. 1995/25349, although it is disclosed to obtain a high resistance element by using a MOSFET in a weak inversion region, International Publication No. 1995/25349 does not disclose or implicitly describe, in any way, a specific configuration showing how to apply the high resistance element to a charge detection circuit.
Further, according to the research conducted by the inventor of the present invention, because a resistance value of a MOSFET in a weak inversion region changes exponentially in accordance with factors such as an oxide film capacitance of the MOSFET, a threshold voltage, a temperature, and the like, the resistance value is extremely susceptible to a variation in processing in a sequence of the MOSFET manufacturing processes, and changes in a power source voltage and a temperature. In addition, the resistance value of the MOSFET in the weak inversion region changes exponentially in accordance with not only a change in a gate voltage but also with changes in drain and source voltages.
As such, in the case where a MOSFET is operated in a weak inversion region and is applied as a pseudo resistance element to a charge detection circuit, an adjustment circuit for the gate voltage is additionally required to adjust a resistance value of the MOSFET, that is, a pseudo resistance. Further, because the pseudo resistance changes when a voltage between the drain and the source of the MOSFET changes, the MOSFET exhibits excessively nonlinear characteristics as a resistance element and, therefore, it waveform distortion is generated in its output signal with a change in the power source voltage, or the like.